1. Field of the Invention
The present invention relates to the improvement of an electrode catalyst layer which attempts to improve the performance of an MEA (Membrane Electrode Assembly) of a solid polymer electrolyte membrane fuel cell, and more particularly to a technique of optimizing the dimensional specification of an electrode constituting material for reducing a reaction loss at an electrode.
2. Description of the Related Art
In the related art, Pt catalysts and solid polymer electrolytes have been used as basic constituent materials for electrode catalyst layers for solid polymer electrolyte membrane fuel cells. While the electrode catalyst layer is an important constituent structure which governs the power generating reaction, since there is caused a loss associated with a reaction referred to as an activated excess voltage, in order to reduce the loss, better combinations thereof have been attempted to be found on an try-and-select basis by preparing variously kinds of Pt catalysts and polymer electrolytes, mixing and distributing amounts thereof and mixing methods.
As one of related arts to the preparation of electrode layers of the solid polymer electrolyte membrane fuel cells, there is known, for example, a fuel cell in which an electrode and a solid electrolyte membrane are prevented from drying by making a gas diffusing electrode hydrophilic so as to make efficient use of water produced from a reaction at the air electrode to thereby improve the cell output characteristics (refer to JP-A-6-275282 (Pages 3 to 4, FIG. 1))
However, since no basic direction for reducing the loss during the electrode reaction has not yet been found, it has resulted that various kinds of catalysts and various combinations of polymer electrolytes are prepared on a try-and-select basis to select the best among what have been tried. Consequently, there is caused a problem that nobody knows whether or not what is so selected is optimal. As a result, despite the fact that lots of efforts have been made on the try-and-select basis over many years, there has been provided no rapid improvement in the performance.
It is a common knowledge among those skilled in this technical field that the surface area of the catalyst such as platinum only has to be increased in order to improve the performance. Therefore, it follows that in case the amount of platinum that is used is the same, the surface area can be increased by reducing the particle diameter of platinum used. In reality, however, the try-and-select approach has not always resulted in a rapid improved in the performance.
In addition, while the amount of platinum used has been tried to be reduced since it is expensive, no sufficient performance has been obtained.